metamorphic core complex

metamorphic core complex The term ‘metamorphic core complex’ was first introduced in the early 1970s to describe occurrences of metamorphic and igneous rocks within North American Cordillera that have a very distinct structural setting. Metamorphic core complexes are large domal features that consist of three elements: a gneissic core, overlain by a low-angle extensional detachment (with dips between 0° and 20°), in turn overlain by supracrustal rocks. The detachment (also termed décollement) and the gneissic core are domal or broadly antiformal in shape. The gneissic core is commonly referred to as the lower plate or footwall and the supracrustal rocks as the upper plate or hangingwall (Fig. 1). The gneissic core is normally composed of high-grade gneisses, which commonly contain eclogites. The rocks are older than the detachment above them, and Precambrian gneisses can thus be involved in a Tertiary core complex. However, syn-extensional granitoid plutons commonly occur in the lower plate and may be deformed by the extensional detachment (Fig. 1). The upper plate can comprise sediments, basement slivers, and syn-extensional volcanic and sedimentary rocks; the sediments commonly include continental basin infill. High-angle normal faults commonly occur in the upper plate, rooted into the low-angle detachment. The extensional detachment itself is usually a knife-sharp contact, underlain by a wider zone of brittle deformation containing fault gouge, fault breccia, and chlorite breccia. Below this there is an even wider zone of mylonite and mylonitic gneiss. This zone can be several kilometres thick.

Despite the low angle of the detachment, it differs from a conventional thrust fault because it puts younger, low-grade rocks over older, high-grade ones. The metamorphic core complex is thus formed as mid-crustal rocks are exhumed by tectonic unroofing in an extensional setting. In the classic examples of Utah, Nevada, and Arizona, the detachments are of mid-Tertiary age, and there is a Tertiary overprint on the high-grade metamorphic cores, for example resulting in local mylonite bands. The extent to which metamorphism or magmatic activity or infiltration of meteoric water accompany the development of the detachment and the emplacement of the core complex have been subjects of much controversy.

More than 30 core complexes are now known to occur in the North American Cordillera in a zone stretching from Arizona to the Canadian Rockies, and hundreds more have been recognized in orogenic belts elsewhere; for instance, in the Aegean, Turkey, the European Variscan, Papua New Guinea, and Egypt. They range in size from about 500 km² in the Aegean to the vast Western Gneiss Region (about 35 000 km²) in the Scandinavian Caledonides of west Norway.

Core complexes have been formed throughout the Earth's history: Pliocene–Pleistocene core complexes (possibly still active) occur in the D'Entrecastaux islands off Papua New Guinea, and Archean core complexes have been described in the Slave Province of the Canadian Shield and in the granite–greenstone belts of the Pilbara in Western Australia. It is likely that many more metamorphic core complexes remain to be discovered.

The detachment fault and the underlying shear zone are the result of a progressive evolution. The deep and hot part of the shear zone (at a depth of 20 km or more) initially deforms in a very ductile manner, resulting in a wide zone of mylonitic gneiss. During further extension, the lower plate is pulled towards higher regions and brought against the cooler upper plate. Ductile deformation is localized in narrow shear zones, cross-cutting the mylonitic gneiss. Continuing extension cools the shear zone further and drags the rocks through the brittle–ductile transition; the ductile structures are locally overprinted by cataclasis and brecciation. This overall retrogressive evolution results in a detachment zone in which ductile and brittle structures occur close together, but in which the latter will always have overprinted the former.

As mentioned above, the extensional detachments juxtapose younger, low-grade rocks on top of older, high-grade rocks. Mineral cooling ages (e.g. K–Ar and Ar–Ar ages) in the lower plate tend, however, to be younger than in the upper plate, because the lower plate cools later than the upper plate. A significant metamorphic break commonly occurs across the detachment, with low-pressure rocks in the upper plate and high-pressure rocks in the lower plate. Metamorphic core complexes are thus also effective vehicles for the exhumation of high-pressure rocks.

The recognition that metamorphic core complexes and their low-angle detachments were the result of continental extension was hampered for a time because rock mechanics predicts that it is implausible for extensional faults to form at such low angles. In addition, very few earthquakes have been recorded that had their origin in low-angle extensional faults. This apparent contradiction was explained when it was suggested that high-angle normal faults may rotate late during their evolution into low-angle normal faults, either by ‘domino-faulting’ or by the rolling hinge model, whereby a steep fault is dragged towards the surface and bent into a lower dip. Many observations of core complexes are, however, incompatible with these models, and it appears that many low-angle detachments were initiated and developed along low-angle faults: a typical example of theory being incompatible with geological observations. Alternative explanations for initiation and development along low angles have invoked high fluid pressures along the detachments or pre-existing anisotropies, as represented by the mylonitic gneiss, that were produced by ductile deformation deeper in the crust. These two mechanisms (possibly in combination) may create a weakness along which brittle movement can take place, even at low angles and with little seismic activity.

Despite the fact that continental extension often leads to subsidence, metamorphic core complexes are commonly elevated with respect to their surroundings. The high topography of individual core complexes (the D'Entrecastaux islands have an elevation of 2.5 km above sea level, but are only 20–30 km across) is the result of isostatic rebound of the lower plate as the overburden of the upper plate is progressively removed. The high topography of larger regions that contain a series of core complexes, such as the North American Cordillera, may, however, be better explained by removal of dense lithospheric mantle underneath the orogenic belt that resulted in uplift of the region as a whole and may also have been the cause of extension.

Metamorphic core complexes are sites of substantial continental extension and thinning of continental crust. All metamorphic core complexes occur in orogenic settings, that is, in sites of previously over-thickened crust, and the extension always post-dates the thickening. This post-thickening extension can be explained by orogenic (or extensional) collapse or by ‘subduction roll-back’. The extension that produces core complexes is not necessarily related to continental break-up, although later continental rifting may reactivate the extensional detachments, as may have been the case in west Norway during the opening of the North Atlantic.

M. Krabbendam and and Bruce W. D. Yardley

Bibliography

Coward, M. P., Dewey, J. F., and Hancock, P. L. (eds) (1987) Continental extensional tectonics. Geological Society of London, Special Publication No. 28.
Crittenden, M. D., Coney, P. J., and Davis, G. H. (eds) (1980) Cordilleran metamorphic core complexes. Memoir, Geological Society of America No. 153.
Hill, E. J.,, Baldwin, S. L.,, and and Lister, G. S. (1992) Unroofing of active metamorphic core complexes in the D'Entrecasteaux Islands, Papua New Guinea. Geology, 20, 907–10.